This application is based on Japanese Patent Application Nos. 2001-232035 filed on Jul. 31, 2001, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates in general to an apparatus for controlling a shifting action of an automatic transmission for an automotive vehicle, and more particularly to techniques for operating a hydraulically operated frictional coupling device at a relatively low temperature of a working fluid, in a manner similar to that at a normal temperature of the working fluid.
2. Discussion of Related Art
For an automotive vehicle, there is known an automatic transmission of a type arranged to be shifted by a shifting action of a hydraulically operated frictional coupling device such that the pressure of a pressurized working fluid to be supplied to the frictional coupling device is regulated by a pressure regulating valve under the control of an electronic control device of a shift control apparatus. The shift control apparatus is arranged such that the pressurized fluid whose pressure is regulated by the electronic control device according to a predetermined control program is directly supplied to the hydraulically operated frictional coupling device after the automatic transmission is commanded to effect a shifting action. This arrangement permits a comparatively high degree of control accuracy of the engaging action of the frictional coupling device. JP-A-2000-9221 discloses an example of such a shift control apparatus for a vehicle automatic transmission, wherein the frequency of a drive signal to be applied to a solenoid-operated valve such as a linear solenoid valve provided to apply a hydraulic signal for controlling the pressure regulating valve is increased when the temperature of the working fluid is lowered, so that the engaging action of the frictional coupling device is controlled with a relatively high response even when the temperature of the working fluid is relatively low.
A known shift control apparatus for an automatic transmission is arranged such that the hydraulic signal for controlling the pressure regulating valve, which hydraulic signal is controlled by the electronic control device, is applied from the solenoid-operated valve to the pressure regulating valve through a switching valve which is operated in response to a command to shift the automatic transmission. Alternatively, the known shift control apparatus is arranged such that the pressurized fluid whose pressure is regulated by the pressure regulating valve is supplied to the hydraulically operated frictional coupling device through such a switching valve. Usually, the switching valve is a shift valve which is switched to effect a selected shifting action of the automatic transmission, according to a hydraulic signal generated from the solenoid-operated valve.
In the known shift control apparatus, the switching action of the switching valve tends to be delayed due to an increase in the viscosity of the working fluid with a decrease in the temperature of the working fluid. Where the pressure of the pressurized fluid to be supplied to the frictional coupling device is regulated according to the predetermined control program immediately after the moment at which the automatic transmission is commanded to be shifted, a delay in the switching action of the switching valve causes a delayed application of the controlled fluid pressure to the frictional coupling device, and an accordingly delayed shifting action of the frictional coupling device. Accordingly, the delay in the switching action of the switching valve causes deterioration of control accuracy of the shifting action of the frictional coupling device due to a low control response of the fluid pressure applied thereto, resulting in a considerable shifting shock of the automatic transmission.
The present invention was made in the light of the background art discussed above. It is therefore an object of the present invention to provide an apparatus for controlling a vehicle automatic transmission, which apparatus permits accurate control of an engaging action of a hydraulically operated frictional coupling device and is capable of effectively reducing a shifting shock of the transmission, irrespective of a varying temperature of the working fluid used for operating the frictional coupling device.
The object indicated above may be achieved according to the principle of this invention, which provides an apparatus for controlling a vehicle automatic transmission having a hydraulically operated frictional coupling device operable to effect a shifting action of the automatic transmission, a switching valve operable according to a shifting command to effect the shifting action, and a pressure regulating valve operable to control a pressure of a working fluid to be supplied to the frictional coupling device through the switching valve to effect the shifting action, the apparatus comprising: (a) a fluid temperature detector operable to detect a temperature of the working fluid; and (b) delaying means for delaying a moment of initiation of an operation of the pressure regulating valve to control the pressure of the working fluid to be supplied to the frictional coupling device for effecting the shifting action, such that the above-indicated moment of initiation is delayed depending upon the temperature of the working fluid detected by the fluid temperature detector.
In the transmission shift control apparatus of the present embodiment constructed as described above, the delaying means is arranged to delay the moment of initiation of an operation of the pressure regulating valve to control the pressure of the working fluid for effecting the shifting action, such that the moment of initiation of the operation of the pressure regulating valve is delayed on the basis of the temperature of the working fluid as detected by the fluid temperature detector, so that the operation of the pressure regulating valve to control the fluid pressure to be applied to the frictional coupling device is initiated when the switching valve has been completely switched to a position corresponding to the operating position of the automatic transmission to be established by the shifting action. Accordingly, the control of the fluid pressure in the frictional coupling device is initiated at an appropriate point of time determined depending upon the temperature of the working fluid, making it possible to effectively reduce a shifting shock of the automatic transmission, irrespective of a variation in the temperature of the working fluid, that is, irrespective of a variation in the viscosity of the working fluid.
According to one preferred form of the present invention, the apparatus further comprises delay time determining means for determining a delay time by which the moment of initiation of the operation of the pressure regulating valve to control the pressure of the working fluid is delayed, such that the delay time is increased with a decrease in the temperature of the working fluid detected by the fluid temperature detector. In this case, the delaying means delays the moment of initiation of the operation of the pressure regulating valve, by the delay time which is determined by the delay time determining means, so as to be increased as the detected working fluid temperature is lowered, so that the operation of the pressure regulating valve to control the fluid pressure in the frictional coupling device to effect the shifting action of the automatic transmission is initiated when the switching valve has been switched to the position corresponding to the position of the automatic transmission to be established by the shifting action. Thus, the delay time determining means permits accurate control of the releasing or engaging action of the frictional coupling device to effect the shifting action, making it possible to effectively reduce the shifting shock of the automatic transmission, irrespective of a variation in the temperature of the working fluid.
In one advantageous arrangement of the above-indicated preferred form of this invention, the shifting action of the automatic transmission is effected by a releasing action of a first frictional coupling device and an engaging action of a second frictional coupling device, which releasing and engaging actions take place substantially concurrently with each other, and the delaying means delays the moment of initiation of the operation of the pressure regulating valve to control the pressure of the working fluid to be supplied to the first frictional coupling device to effect its releasing action for effecting the shifting action of the automatic transmission. In this case, the delay time is a length of time from a moment of generation of the shifting command to the moment of initiation of the operation of the pressure regulating valve to initiate the releasing action of the first frictional coupling device. Accordingly, the accuracy of control of the releasing action of the first frictional coupling device is improved. Further, the engaging action of the second frictional coupling device is effected in timed relation with the releasing action of the first frictional coupling device, so that those releasing and engaging actions of the two frictional coupling devices can be controlled with a high degree of accuracy, making it possible to further effectively reduce the shifting shock of the automatic transmission, irrespective of the varying temperature of the working fluid.